The long-term goal of this project is to understand why infants' responses to sound change as they get older. Specifically, the aims are to evaluate how the acoustic properties of the ear, maturation of the nervous system, and maturation of factors such as attention and motivation affect auditory behavior during development. Three questions are addressed. First, why do infants require higher intensities than adults do to detect a sound? Measures of the acoustic properties of the infant ear, evoked potential measures of neural function, and behavioral detection performance will be examined to answer this question. Second, why do infants seem unable to follow rapid changes in sounds over time? Behavioral performance in detecting gaps in noise and in detecting amplitude modulation of sound will be compared to two different evoked potential measures of the same abilities. Third, why do young infants seem less able than adults to respond selectively to one frequency component of a complex sound? Tuning curves, a measure of auditory frequency resolution, will be measured behaviorally and using the auditory brainstem response in the same infants to address this issue. The logic behind all of the proposed experiments is that a combination of measures at different levels of analysis can provide considerably more information about the mechanisms responsible for development than any of the measures alone. By evaluating both sensory and nonsensory contributions to auditory response, this approach will provide information that is important in designing effective habilitation for hearing-impaired children, as well as helping to explain the course of normal auditory development.